Digitizer tablets are well known in the art. In one popular type, the pointing device comprises a coil in a cursor or stylus configuration which is positioned by a user over a tablet surface in which is embedded a conductive wire grid extending in two coordinate directions. In one mode, the pointing device coil is energized to electromagnetically induce signals in the grid wires. In another mode, the grid wires are energized to electromagnetically induce signals in the pointing device coil. In both modes, the grid wires are sequentially addressed, either one at a time or in groups, to provide an output analog voltage in timed relation to the grid addressing. The output voltage with increasing time reaches a maximum, passes through zero, and then reaches a minimum. When addressing begins, a counter is activated which keeps track of the particular grid line addressed. A stop signal for the counter is generated when the zero crossover is determined. The count value determines the location of the pointing device with respect to the two or three closest grid wires. Examples of patents describing in more detail this type of digitizer are Kamm et al. U.S. Pat. Nos. 3,904,822; Ioanau 3,873,770; and Zimmer 4,368,351, whose contents are hereby incorporated by reference. BYTE, January 1989, pages 162-174, gives a general description of such devices and their performance.
Because of the speed of the signal processing, it is possible to scan the grid conductors a number of times before the user has changed the position of the pointing device. This allows the coordinate pair outputted to be the average of a number of scans. This eliminates aberrant signals and improves accuracy. By "scan" is meant, as will be clear from the foregoing description, applying to the grid conductors, usually sequentially, a high-frequency energizing signal 10 (commonly 10 KHz to 100 KHz), and processing the signals induced in the pointing device, or applying the high-frequency energizing signal to the pointing device, and scanning the grid conductors (usually sequentially) to collect the induced signals for subsequent processing. A typical scan rate is 100-200 Hz. This means that 100-200 times per second, the grid/pointing device are activated, induced signals collected and processed to determine a coordinate position. Averaging of points determined is typically used to protect against inevitable erratic readings. Due to the relatively low scan rate, and to avoid reducing the device response time (typically up to 150 outputted or reported coordinate pairs per second), heavy averaging type filters are typically not used but instead running average type filters. With the latter, each new determined point is averaged with a previous average and a new output reported each time, which maintains a constant report rate. Various degrees of filtering are obtained by variable weighting in the averaging process of the previous average and the new point. It is known to heavily weight the previous average compared with the new point (as high as 100:1) to reduce noise and jitter, usually random fast changes in reported coordinates, represented on the display by fast erratic cursor movement.
A problem has been observed during the operation of such tablets. It has been observed that, from time to time, though the pointing device is held in one position on the tablet's working surface, the cursor displayed on the display device connected to the computer that is converting the pointing device position, wanders, i.e., its position is not fixed as it should be but exhibits a slow cyclic movement, herein referred to as "wobble".